The present invention relates to an activated carbon and a process for producing the same.
The activated carbon of the present invention is very excellent in terms of fixation of titanium dioxide to the activated carbon particle surfaces, and thus is an activated carbon in which titanium dioxide fine particles exist on and/or in an activated carbon particle surface without plugging up pores in the activated carbon particle. Also, the activated carbon of the present invention can drastically enhance the removing efficiency of harmful substances in water or gas under application of ultraviolet rays or the sunlight, so that it can be advantageously used for the purification treatment of service water, sewage, exhaust gas, etc., removal of malodor, and other purposes.
Generally, an activated carbon has an excellent absorptivity owing to the large specific surface area thereof and is popularly used for adsorption removal of the harmful substances in water or gas.
Recently, pollution of water, sea and atmospheric air by domestic and industrial wastes is spreading on the global scale. Eutrophication of lakes and rivers by domestic waste water including synthetic detergents and such, contamination of ground water or water supply sources by the organic solvents used in the semiconductor manufacturing plants or laundries, and pollution of water by efflux of agricultural chemicals used in the golf courses, etc., are the typical examples of exigent pollution problems.
Presently, activated sludge process is most popularly employed for waste water treatment, but this process is subject to strict regulations in relation to a temperature, pH, gas atmosphere, toxicity, etc., since it uses microorganisms. This process is also at a disadvantage in that it is almost ineffective for decomposition removal of agricultural chemicals, organic solvents (including halides), surfactants and the like. For the treatment of the organic substances which are biologically hard to decompose, there are available several effective methods such as a chlorine treatment method, ozone treatment method, incineration method and activated carbon adsorption method. The chlorine treatment method involves the problem that it causes a problem of residual chlorine due to excess feed, and chlorine may be reacted with the organic substances contained in water to be treated to produce organic halides such as trihalomethanes, etc. which are known to have carcinogenecity.
Recently, the ozone treatment method comes into the spotlight as an advanced water purification treatment method at the water purification plants and other similar facilities, but this method also has the problem of high cost of equipment and operation. The incineration method is impractical for the treatment of dilute solutions. The activated carbon adsorption method is a very effective method, but this method is still unsatisfactory in its adsorption removability of organic halides and is not effective for all of harmful substances in water.
The activated carbon adsorption method is also useful for removal of harmful substances in gas such as atmospheric pollutants and malodorants. Generally, it is required that the adsorption technique designed to be applied to the pollutants in gas is effective to low-concentration gaseous substances in the presence of steam or carbon dioxide. An activated carbon is used for removing of various kinds of organic and inorganic compounds under such conditions. The activated carbon for use in a gaseous phase has a large specific surface area as well as a porous structure with small pore size, and the adsorptive affinity thereof for low-concentration gases is high. Also, because of the hydrophobic surface thereof, activated carbon is small in adsorptive affinity for steam and is capable of removing harmful gaseous substances or malodorants existing in the gas, especially organic compounds, at high efficiency. However, there are the kinds of gaseous substances for which the adsorptive affinity to the activated carbon is weak, and thus activated carbon is not almighty for adsorption removal of harmful substances.
On the other hand, ever since the utilization of energy of light for direct decomposition of water by semiconductor photoelectrodes composed of crystals of titanium dioxide (Honda-Fujishima effect) has been realized in 1969, many and enthusiastic studies have been made on photo-catalysts represented by titanium dioxide in technical fields as a potential means for converting energy of light into chemical energy. Such a reaction is called photocatalytic reaction which proceeds with the aid of light, that is, even if a catalyst is present in the reaction system, the reaction does not proceed without application of light. This photocatalytic reaction has close relation with the ordinary catalytic reactions and photochemical reactions, but on the other hand has distinct differences from these ordinary reactions. The driving force of the ordinary catalysts is heat, and the transferring rate from the reaction system to the production system is varied by the presence of a catalyst. Therefore, the role of the catalyst is to control the reaction rate till reaching the equilibrium state specified by a temperature, pressure and other conditions of the system concerned, and the reaction attainable is restricted to one which can proceed thermodynamically. On the other hand, in a photochemical reaction, light is absorbed into the reaction system to cause a change of electronic state and chemical bond of the reactants so that the reaction system is converted into the production one, and as a result, a reaction that can never be brought about by a thermal reaction such as an ordinary catalytic reaction, can be realized.
In the photocatalytic reaction, on the other hand, the catalyst which has been brought into an electroncially excited state on absorption of light, acts to the reaction system, thereby proceeding the reaction only on the catalyst surface. This electronically excited state of the catalyst, as in the case of photochemical reaction, corresponds to a non-equilibrium state in which the electron temperature alone arose excessively, and consequently the reaction can proceed even under a mild condition wherein the reaction is considered thermodynamically impossible. This testifies that there are the cases where the principle in the ordinary catalytic reactions that "catalyst does not change the equilibrium of a chemical reaction" does not hold true, and this fact is characteristic of photocatalytic reactions.
Generally, a photocatalytic reaction comprises (1) a photo-excitation step in which the semiconductor is excited on absorption of light to generate electron-hole pairs, (2) a charge separation and transfer step in which the generated electrons and holes are transferred to the surface by virtue of potential gradient in the semiconductor particles and diffusion thereof, and (3) a surface reaction step in which the holes and electrons which moved to the surface, cause electron transfer with the compound adsorbed on the catalyst to induce a redox reaction.
Several techniques utilizing the characteristics of an activated carbon and titanium dioxide have been proposed. For instance, Japanese Patent Application Laid-open (KOKAI) No. 6-315614 discloses a method for removing pollutants by use of a mixture of titanium dioxide and an activated carbon. With the methods disclosed in Japanese Patent Application Laid-open (KOKAI) No. 6-315614, however, since the activated carbon and titanium dioxide exist separately from each other, it is difficult to make use of the potency of titanium dioxide to the maximal degree. It is also difficult to use the conventional purifying apparatuses directly in the form as they are stand.
As the activated carbon in which titanium dioxide fine particles exist on and/or in an activated carbon particle surface, it is well known a white activated carbon disclosed in Japanese Patent Application Laid-open (KOKAI) No. 4-256436 and a colored-activated carbon disclosed in U.S. Pat. No. 3,746,655. However, the former white activated carbon is an activated carbon in which titanium dioxide is adhered on the surface thereof to obtain a white colored-activated carbon. Therefore, if the surface of the activated carbon is covered with titanium dioxide until the surface color becomes white, the adsorption ability of the activated carbon is remarkably decreased. The later colored-activated carbon is an activated carbon in which a binder resin containing any kind of pigment is applied and adhered. Since the surface of the colored-activated carbon is covered by the binder resin, the adsorption ability of the activated carbon is also remarkably decreased. Further, when the colored-activated carbon is utilize as the photo-catalyst of titanium dioxide, the titanium dioxide comes off the activated carbon by the decomposition of the binder resin. Still further, in both Japanese Patent Application Laid-open (KOKAI) No. 4-256436 and U.S. Pat. No. 3,746,655, there is no description of photo-catalyst action.
As result of the present inventors' studies for overcoming the above-mentioned problems, it has been found that (i) by pulverizing coal, granulating the pulverized coal, carbonizing and activating the granules, titanium dioxide being added to the coal before the activation thereof, or (ii) by finely pulverizing a carbonaceous material, mixing with a binder, granulating the resultant mixture, hardening and carbonizing the granules and then activating the carbonized substance, titanium dioxide being added to the carbonaceous material before the activation, (1) quite surprisingly TiO.sub.2 can remain as anatase-type or rutile-type TiO.sub.2 having a photocatalytic action without producing a by-product such as Ti.sub.n O.sub.2n-1, etc. even when TiO.sub.2 is placed in a strongly reducing atmosphere of the carbonization and further even if a steam activation (in which H.sub.2 is generated and carbon exists therearound) is conducted in the activation step; (2) titanium dioxide is fixed fast on and/or in the activated carbon particle surfaces without plugging up the pores thereof, and minimizes separation of titanium dioxide; (3) further, in this case, it is possible to select as the starting carbonaceous material from various materials including coconut shell, coke, charcoal and coal, and to freely control the shape of the produced activated carbon; and (4) the obtained activated carbon in which a pertinent amount of titanium dioxide is fixed on and/or in the surface of activated carbon particle, that is, a lightness L value (Hunter) is not more than 50, is remarkably improved in its ability to remove the harmful substances in water or gas under application of ultraviolet rays or the sunlight. The present invention has been attained on the basis of this finding.